1. Field of the Invention
The present invention relates to an optical pickup and a hologram laser designed for use in reading and writing of signals in optical disks such as CD, CD-R/RW, DVD, and DVD±R/RW, and more particularly to an optical pickup and a hologram laser that are suitable for reading out information recorded in an optical disk having a plurality of information recording surfaces.
2. Description of the Related Art
Conventionally, an optical disk called “CD family disk” has been used in which reading and writing of signals are performed with use of a semiconductor laser device having an emission wavelength of 780 nm as a light source. In the case of the CD family disk, tracking servo control is generally exercised by means of the so-called 3-beam method with which a diffraction grating is required.
And also, in recent years, an optical disk called “DVD family disk”, which is capable of recording larger quantities of information, has been coming into wider and wider use. In the DVD family disk, a red-color semiconductor laser device having an emission wavelength of 630 to 690 nm is employed as a light source for reading and writing signals. In the case of the DVD family disk, tracking servo control is exercised by means of the Differential Phase Detection method (DPD method). Moreover, in order to increase information recording capacity, an optical disk having a plurality of information recording surfaces is employed. In this case, there occurs a phenomenon in which reflection light comes from other information recording surfaces than a given information recording surface kept in an information-reading state. In order to cope with the resultant reflection light, certain countermeasures must be taken (for example, refer to Japanese Unexamined Patent Publications JP-A 7-129980 and JP-A 9-161282).
In addition to that, CD-R and CD-RW (hereafter referred to as “CD-R/RW” on the whole), DVD-R, DVD-RW, DVD+R, and DVD+RW (hereafter referred to as “DVD±R/RW” on the whole) have been used as information recording media. These recordable optical disks are each provided with guide grooves running along the information recording track. Since the guide groove is not a simple groove, but is configured as a wobbled groove, tracking servo control is exercised by means of the PP method. In this case, however, a DC offset may be caused in a PP signal due to a tilting of the disk, which results in the recording accuracy being deteriorated. In order to cancel the DC offset, the Differential Push Pull method (DPP method: a diffraction grating is required) is adopted. Since, in particular, an optical pickup designed for a DVD family optical disk is required to deal also with reading operations on a CD family disk, even if the DPP method using three light beams is adopted, there is no need to increase the number of optical elements.
FIG. 13 is a view schematically showing the structure of an optical pickup of standard design. The optical pickup 1 includes a hologram laser 2, a collimator lens 3, a raising mirror 4, and an objective lens 5. The optical pickup 1 serves to read out recorded information from an information recording surface 7 of an optical disk 6, and to perform information recording. The hologram laser 2 is designed as a single unit composed of a combination of a semiconductor laser chip acting as a laser light source; a photodiode acting as a signal-detection light receiving element; a hologram for deflecting light returned from the optical disk 6 to the light receiving element; and a diffraction grating for splitting laser light into three light beams.
FIG. 14 is a view of one conventional example, illustrating a hologram 8 pattern of a hologram element, a configuration of a light receiving domain in a light receiving portion of a light receiving element 9 corresponding to the hologram pattern, and a method for reading out a signal. Also shown in FIG. 14 is the plane-wise positional relationship between the hologram 8 and the light receiving element 9. The plane is perpendicular to the optical axis. Further shown in FIG. 14 are patterns of light transmitted through or diffracted at the hologram 8 pattern, as observed on the light receiving domain. Since the light receiving element 9 is located farther away from the hologram 8 than the focal point, the laser light pattern observed on the light receiving domain dose not coincide with the hologram 8 pattern. By contrast, if the light receiving element 9 is located closer to the hologram 8 than the focal point, the laser light is proportional in pattern to the hologram 8.
In the conventional example such as shown herein, the knife-edge method is applied to focusing control, and the DPP method is applied to tracking control. In order to apply the knife-edge method to focusing control, a difference in output signal between the domains D5 and D6 is obtained by using half of a main beam. D4 and D7 are provided to cancel a DC offset which is caused to a focusing control signal by the light reflected from a different information recording surface from the one kept in an information-reading state (for example, refer to Japanese Unexamined Patent Publications JP-A 9-161282 and JP-A 2000-57592).
As shown in FIG. 14, the hologram 8 is divided into three regions by a straight line running in a direction parallel to a direction equivalent to the track direction of the optical disk and another straight line running in a direction perpendicular to the direction equivalent to the track direction, thereby constituting a three-division hologram. Note that “the direction equivalent to the track direction” refers to a direction in which, for example, three light beams lined up in the track direction on the optical disk 6 as shown in FIG. 13 are rearranged in an array on the hologram 8 after being converged through the objective lens 5 and the collimator lens 3 or reflected from the raising mirror 4.
Hereafter, the DPP method will be described in detail with reference to FIGS. 15 through 17. As is widely known, on the information recording surface 7 of the optical disk 6 are arranged information-bearing pits in the track direction. The light converged on the pit is reflected from and simultaneously diffracted at the pit. Of the light from the pit, the undiffracted light component is allowed to pass through the objective lens 5, whereas the diffracted light component is partly rejected at the objective lens 5. The overlapping portion between the diffracted and undiffracted light components appears to brighten and darken due to interference effects, so that bright and dark portions are created as shown in FIG. 15. Then, the main beam is split into two beam components by a center line running in the track direction, and a difference in signal between the beam components is obtained to generate an MPP signal (Main Push Pull signal) In this way, it will be found that no signal difference is observed when the main beam is present right above the track. By exploiting this fact, it is possible to control the light converged through the objective lens 5 to be located above the track at all times. That is, tracking control can be achieved properly.
In regard to a sub beam, by obtaining an SPP signal (Sub Push Pull signal), namely, a difference between a light beam of + first order and a light beam of − first order, a push pull signal can be generated. Moreover, by obtaining a DPP signal, namely, a difference between MPP and SPP signals, a tracking control signal of higher accuracy can be generated. As shown in FIG. 17A, in terms of the circuit, an SPP signal of lower intensity is amplified by K times relative to an MPP signal to make its intensity coincide with the intensity of the MPP signal. Thereby, as shown in FIG. 17B, a DPP signal can be generated. In a 3-beam-generating diffraction grating 10 which is used in the way as shown in FIG. 16A, phase shift regions are secured for the purpose of facilitating adjustment of the position of the sub beam (the position with respect to the track) (for details, refer to Japanese Unexamined Patent Publication JP-A 2001-250250, for example).
Since pit information is included in every main beam, as shown in FIG. 14, the signals outputted from all the light receiving domains on which the main beam is incident are summed up (D2+D9+(D4+D5+D6+D7)) to obtain an RF signal (information signal). This makes it possible to maximize the signal intensity.
As described heretofore, the DPP method is necessary to achieve reading and writing on a disk for recording purposes. However, in this case, the following problem is raised. For example, when a double-layer disk having a plurality of information recording surfaces 7 is subjected to information reading operations, a signal from the information recording surface kept in a non-reading state finds its way into a tracking signal-detection SPP signal as a DC offset. In general, in the case of reading out information from a single-layer information recording surface with a push pull signal such as a focusing error signal, by changing the distance between the objective lens 5 and the information recording surface 7, an S-shaped curve is obtained, and the signal is converged to zero at the focused focal point. Meanwhile, in the double-layer disk, when the light returned from the other information recording surface than the one kept in an information-reading state is also received simultaneously, due to the influence of the return light, the signal fails to converge to zero at the focused focal point. This leads to occurrence of a DC offset.
In an attempt to prevent occurrence of an offset, JP-A 7-129980 and JP-A 9-161282 proposed the following techniques. According to the former, an HOE (Hologram Optical Element) is employed in which a plurality of light receiving domains are arranged substantially perpendicularly to the track direction so as to cause astigmatic aberration in diffracted light. In this case, the spot center on the photo detector can be prevented from deviating from the dividing line by canceling out the influence of the change in hologram diffraction angle resulting from wavelength fluctuations. This makes it possible to prevent an offset from occurring in a tracking error signal or a focusing error signal. However, the HOE necessitates two pieces of hologram elements.
According to the latter, to prevent an offset from occurring in a focusing error signal FES, in addition to two main light receiving domains, an auxiliary light receiving domain is provided. The auxiliary light receiving domain detects return light which expands out of the two main light receiving domains in a greatly defocusing state. Since FES curves derived from a plurality of layers do not interfere with one another, no offset is produced. In this case, however, it is impossible to avoid causing an offset in a tracking error signal.